The present invention relates to a dimmer for fluorescent lighting systems, and more particularly to a dimmer which controls the AC current from the power line to vary the output intensity of a fluorescent lamp having a magnetic ballast.
One way of controlling escalating energy costs is by limiting energy consumption. In a modern office building, the principle energy consumers are lighting and heating and cooling. To conserve energy, the thermostat is xe2x80x9cturned backxe2x80x9d and the lighting is reduced during non-office hours. Reducing the energy consumption from lighting essentially involves dimming the lamps or turning off selected lamps. To conserve energy during non-office hours, most banks of lamps on a floor are turned off, with a few banks of lamps being left on to provide some lighting for security. The other approach to conserving energy consumption involves dimming the fluorescent lamps during non-office hours. As a result of being dimmed less power is consumed, while at the same time a minimum light level is maintained for security purposes.
In a typical office building the lighting system comprises banks or groups of fluorescent lamps. A fluorescent lamp is a type of lamp in which light is generated by fluorescence. The most common form of fluorescent lamp comprises a gas-discharge tube which contains a low-pressure gas such as mercury. The inner surface of the tube is coated with phosphor and when a current passes through the tube a discharge results and the ultraviolet radiation produced strikes the phosphor which then emits visible radiation. To start the discharge, i.e. turn on the lamp, the current must be provided at a sufficiently high voltage level, and typically a form of ballast circuit is utilized to produce the discharge current.
Compared to incandescent lamps, fluorescent lamps present special problems with respect to dimming. Various solutions have been proposed for dimming fluorescent lamps, including a magnetic ballast, an electronic ballast, and an electronically tapped voltage transformer.
The magnetic ballast solution produces a high voltage when there is no discharge in the lamp (i.e. the lamp is not conducting) and also feeds a xe2x80x9ccathode heater circuitxe2x80x9d. When the arc (i.e. discharge) starts in the tube, the voltage at the output of the secondary winding on the ballast collapses to a level which is necessary to sustain the arc. The ballast absorbs, i.e. through its inductance, the excess voltage from the power source. There have been several dimmers proposed in the art based on the variation of the voltage controlling the discharge in the lamp, but none of these solutions have achieved any commercial success.
Another type of known dimmer for fluorescent lamps is based on an electronic ballast. The electronic ballast generates a rectified DC voltage from a power source and injects a resonant current into the lamp tube. The resonant current has a relatively high frequency (typically 20 kHz) and as a result special tubes are required for the fluorescent lamps. Each lamp requires an electronic ballast. The electronic ballast is modified for dimming control by providing a variable DC voltage.
In view of the shortcomings with the state of art devices, there remains a need for a dimmer for use with fluorescent and other types of gas discharge lamps.
The present invention provides a current controlled dimmer for fluorescent lamps. The current controlled dimmer generates a feedback controlled current signal output with a waveshape which follows the voltage drive signal for the lamp. By varying the amplitude of the current output signal, the output intensity of the fluorescent lamp can be decreased (i.e. dimmed) or increased (i.e. intensified). According to the invention, the voltage drive signal across the lamp electrodes (i.e. ballast) is kept constant and a constant heating current is maintained so that the lamp can respond almost instantaneously to an increase in the amplitude of the current signal.
In accordance with the present invention, the current signal output is obtained by modulating the AC line (i.e. drive) voltage to generate an AC current signal. The current controlled dimmer utilizes a feedback control loop which applies proportional/integral (PI) control to the PWM control signal to superimpose a fast response (e.g. 2 kHz) over the steady state base chopping rate. Advantageously, this feature eliminates noticeable flicker in the lamp output. The generated AC current signal output has a quasi-sinusoidal waveform which follows the sinusoidal voltage waveform over the range of operation.
In one aspect, the present invention provides an apparatus for controlling the output intensity level of a gas discharge lamp having a magnetic ballast, the apparatus comprises: (a) means for coupling an AC supply voltage to the magnetic ballast for energizing the ballast to produce a discharge in the gas discharge lamp; (b) means for generating an intensity level signal for setting the output intensity level for the lamp; (c) switch means for switching the AC supply voltage to generate an AC current for powering the gas discharge lamp, the switch means being responsive to a chopping control signal for varying the amplitude of the AC current and thereby varying the output intensity of the lamp; (d) controller means for controlling the switch means, the controller means including a pulse width modulator for generating the chopping control signal, the pulse width modulator having means responsive to the intensity level signal for generating the chopping control signal with a duty cycle derived from the intensity level signal.
In another aspect, the present invention provides an apparatus for controlling the output intensity level of a gas discharge lamp having a magnetic ballast or a group of lamps each having a magnetic ballast and being connected to a single protection device such as a circuit breaker or fuse, the apparatus comprising: (a) means for coupling an AC voltage to the magnetic ballast for energizing the ballast to produce a discharge in the gas discharge lamp; (b) means for generating an intensity level signal for setting the output intensity level for the lamp; (c) switch means for switching the AC voltage to generate an AC current for powering the gas discharge lamp, the switch means being responsive to a chopping control signal; (d) controller means for controlling the switch means, the controller means having means responsive to the intensity level signal and including a pulse width modulator for generating the chopping control signal and the chopping control signal having a duty cycle derived from the intensity level signal.
In yet another aspect, the present invention provides a method for controlling the output intensity level of a gas discharge lamp having a magnetic ballast, the method comprising the steps of: (a) applying a voltage to the magnetic ballast for energizing the ballast and producing a discharge in the gas discharge lamp; (b) modulating the voltage to produce an alternating current for powering the gas discharge lamp, the alternating current having a controllable waveshape substantially following a reference signal; (c) generating an intensity level signal from the reference signal for setting the output intensity of the lamp; (d) varying the modulation of the voltage in response to an error signal, the error signal comprising the difference between the intensity level signal and a feedback current signal, so that the output intensity level of the gas discharge lamp follows the reference signal.
In another aspect, the present invention provides a method for controlling the output intensity level of a gas discharge lamp having a magnetic ballast, the method comprising the steps of: (a) applying a voltage to the magnetic ballast for energizing the ballast and producing a discharge in the gas discharge lamp; (b) modulating the voltage signal to produce an alternating current with a variable magnitude for powering the gas discharge lamp; (c) inputting an intensity level signal for setting the output intensity level of the lamp; (d) varying the modulation of the voltage in response to the intensity level signal to change the magnitude of the alternating current and thereby vary the output intensity of the gas discharge lamp.
In another aspect, the present invention provides, an apparatus for controlling the output intensity level of a gas discharge lamp having a magnetic ballast, the apparatus comprises: (a) means for coupling an AC supply voltage to the magnetic ballast for energizing the ballast to produce a discharge in the gas discharge lamp; (b) means for generating an intensity level signal for setting the output intensity level for the lamp; (c) switch means for switching said AC supply voltage to generate an AC current for powering the gas discharge lamp, the switch means being responsive to a chopping control signal for varying the amplitude of the AC current and thereby varying the output intensity of the lamp; (d) controller means for controlling the switch means, the controller means including means responsive to the intensity level signal for generating a chopping control signal with a duty cycle derived from the intensity level signal.
Advantageously, the current controlled dimmer according to the present invention provides the following beneficial features. Current control of the lamp output suppresses flicker which results in a steady light emission from the lamp. The constant light emission, in turn, produces a perceived brighter output even though the lamp is powered at a lower level. Operation at less than full power (e.g. 80%) improves the operating life of the ballast in the lamp by reducing excess heating. Furthermore, the balancing of the current signal also reduces overheating in the ballast and eliminates harmonics. It has been found that the injection of even order harmonics can be particularly detrimental to the longevity of the ballast in a fluorescent lamp. In addition, the slight lag in the current feedback produces a phase advance in the current signal which allows the power factor to be maintained above 0.9.